Apparatus for dehydrogenation of ethylbenzene to styrene

ABSTRACT

The present invention discloses a process and apparatus for improving the catalyst life and efficiency in a gas flow catalyst bed reactor assembly. The reactor comprises an outer reaction vessel, an inner displacement cylinder, and an annular catalyst bed surrounding the displacement cylinder having a top half and a bottom half. Fluid flow improvement is achieved by adding at least one baffle to the top half of the displacement cylinder to improve uniformity of fluid flow in the reaction vessel and across the catalyst bed. Also disclosed is a process for improving fluid flow uniformity in a gas phase reactor comprising an outer reaction vessel, an inner displacement vessel having a top half and a bottom half and a reaction outer surface and an inert inner space, and an annular catalyst bed. The process comprises conducting fluid flow simulations using actual reactor conditions. During simulation, baffles are added on the outer reaction surface of the displacement reactor to improve simulated fluid flow. The baffles are added to the displacement cylinder by entering the inner inert space of the cylinder and attaching the baffles to the reaction outer surface from the inner inert space. The process allows the modification of existing reactors without disassembling the reactor.

BACKGROUND OF THE INVENTION

This invention relates to the field of styrene manufacture and moreparticularly discloses apparatus including reactor vessels for thedehydrogenation of ethylbenzene into styrene monomer.

It is well known in the art of styrene manufacture to react ethylbenzeneover a dehydrogenation catalyst such as iron oxide under elevatedtemperatures in the range of around 1000° F. and at a pressure of about10 to 20 PSIA in order to strip hydrogen from the ethyl radical on thebenzene ring to form the styrene molecule. This is normally done in astyrene radial reactor which also is commonly termed an EB dehydroreactor. The dehydro reactors generally are elongated cylindricalvertical structures of a very large size ranging in diameter from aboutfive to thirty feet or more and in length from about ten to one hundredfeet or more. The normal construction for such a reactor allows forinput of the ethylbenzene gas at an inlet located in the bottom centerof the vertical reactor, whereupon the gas is flowed up through anannular area, passing radially outward through a porous catalyst bed ofiron oxide or other suitable dehydro catalyst, and then passing upwardthrough an outer annular area to exit at the top of the reactor shell.Since the flow of ethylbenzene across the catalyst bed is in a radialdirection, these reactors are sometimes identified as “radial” reactors.

Normally a radial reactor would be sized such that the annular flow areainside the catalyst bed would have some relative proportional value withrespect to the cross-sectional flow area of the inlet pipe deliveringethylbenzene to the reactor. Preferably the annular flow area inside thecatalyst bed would be larger than the cross-sectional flow area of theflow inlet pipe. Because of the extended vertical length of suchreactors, normally the inlet pipe to the bottom of the reactor must comein at a relatively sharp ninety-degree radius and the resulting effectis a side-to-side maldistribution of flow across the reactor vessel.Ideally, the inlet pipe to the reactor would be a straight vertical pipefor a considerable distance prior to entering the reactor, but due tophysical configurations, this is not possible because of the extendedvertical height of the reactor.

Also, due to the nature of flow across the extended vertical length ofthe reactors, switching from longitudinal or axial flow into radial ortransverse flow and then back into longitudinal flow, flow velocitiesacross the catalyst bed from top to bottom vary widely in conventionalreactor vessels, thus resulting in degraded catalyst life in those areasof the reactor with the greatest flow velocities. It has been found byexperimentation and flow velocity measurements that the highest feedvelocity across the catalyst beds in a radial reactor generally occursnear the top of the reactor, and the lowest velocity across the catalystbed occurs near the bottom of the reactor near the inlet pipe. Thisincreased velocity at the top of the catalyst bed and reduced velocityat the bottom of the catalyst bed results in a greatly shortened life ofthe catalyst near the top of the reactor and forces a shutdown of thereactor for catalyst regeneration much sooner than normally desirable.

Accordingly, it is desirable to improve the flow in the reactor both inthe axial and vertical directions. U.S. Pat. No. 5,358,698 to Butler etal. issued on Oct. 25, 1994 and is assigned to Fina Technology, Inc.This '698 patent discloses a method for improving the flow in adehydrogenation reactor by using a displacement cylinder. The disclosureof this patent is hereby incorporated by reference in its entirety.While improvement in fluid flow is achieved by the method taught in the'698 patent, further improvements were needed in order to improve theefficiency of the catalyst.

SUMMARY OF THE INVENTION

The present invention discloses a dehydrogenation reactor vesselapparatus that comprises a displacement cylinder and utilizes specificbaffling on the exterior of such displacement cylinder to reduce thevertical flow differences across the reactor height. The baffles areattached to the displacement cylinder without having to disassemble thereactor. The baffles are attached to the exterior of the displacementcylinder at specific locations to reduce the flow rate in the higherflow rate regions of the reactor. In one embodiment, at least twobaffles are added to the top half of the reactor to allow more uniformfluid flow through the reactor.

In accordance with one embodiment of the present invention, an existingethylbenzene dehydrogenation reactor is retrofitted to improve fluidflow and extend catalyst life. Retrofitting the reactor starts withanalysis of existing reactor condition and catalyst loading. The fluidflow through the reactor is simulated. Once simulated conditions reflectactual operations, fluid flow improvements are simulated. Theimprovements comprise adding baffles to the displacement column atlocations exhibiting higher fluid flow velocities. The location, sizeand number of baffles are determined by simulation to provide as uniforma fluid flow as possible. After simulation, the actual baffles are addedto the outside of the displacement column without disassembly of thereactor. The baffles are preferably added to the top half of the reactorand do not extend more than half the distance from the displacementreactor to the inner wall of the catalyst bed. The process results inoptimization of pressure drop while minimizing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross sectional schematic diagram of the reactorvessel and baffles location in accordance with one embodiment of thepresent invention.

FIG. 2 shows the normalization of fluid flow in one reactor modified inaccordance with the present invention.

DETAILED DESCRIPTION

As has been the trend in the industry, larger reactors for themanufacture of styrene are utilized in order to reduce operating costs.While the present invention is described relative to an ethylbenzenedehydrogenation reactor to form styrene, the invention is applicable toimproving the operations of large reactors with fixed catalyst beds.

FIG. 1 is a schematic cross sectional side view of an EB dehydro reactorvessel 10 having an elongated outer cylindrical shell 11 enclosing aninner cylindrical displacement member 12 located concentrically insidecylindrical vessel 11. Vessel 11 and displacement member 12 aregenerally right circular cylinders, meaning that a cross sectional viewtaken perpendicular to the longitudinal center lines of these twovessels would be circular in shape. Preferably, displacement cylinder 12is located co-axially within vessel 11, meaning that the centrallongitudinal axis of the two cylindrical structures coincide. An inletpipe 13 having a large cross sectional area is connected to a centralinlet opening 14 formed in the bottom of shell 11. Preferably inlet pipe13 is also cylindrical in cross sectional area.

The placement of cylinder 12 within vessel 10 in a coaxial alignmentserves to form an annular catalyst area 18 around the displacementcylinder. A series of optional radially outwardly extending flow bafflesmay be formed on the outer wall of catalyst bed, extending radiallyoutward therefrom to further direct flow of gases flowing through thecatalyst bed and directing them into a radial flow direction, therebypreventing longitudinal flow and further smoothing out flow across thecatalyst bed. Once these catalyst bed baffles are installed, thelocations cannot be changed without long downtime and disassembly of thereactor.

The catalyst bed 18 comprises a concentric cylindrical catalyst shellmade of a perforated or porous inner wall and a similar porous orperforated outer wall. Preferably, the catalyst shell is sufficient tomaximize flow and still retain the dehydro catalyst between the innerand outer walls. Some typical catalysts utilized in the dehydrogenationprocess are those sold by United Catalyst (Styromax series) and byCriterion (such as Versicat series). These may be of the iron oxide typeor other dehydrogenation types of catalysts. The shape and size of thecatalyst particles varies and may have an effect on the fluid flow inthe reactor.

The sizing of the flow areas of the inlet pipe 13 and the annular area21 (inner annulus) between the displacement cylinder 12 and the catalystbed 18 is preferably in the range of about 2 to 1 with annular area 21being approximately twice the value of the cross sectional area of pipe13. Furthermore, the annular area 22 (outer annulus) between catalystbed 18 and vessel 11 is relatively narrow and would not allow sufficientspace for later modifications or work in the area. To perform any workon the interior of the vessel wall 11, the catalyst bed 18 must beremoved. This represents significant cost and long downtime. Outlet pipe5 and opening 6 provide the means for removal of product from thereactor. Baffles 30, 31, and 32 are shown in accordance with oneembodiment of the present invention. These baffles affect the flowthrough the reactor and do not operate to achieve the same function ofany baffles that may be present in the inlet pipe 13 or baffles oncatalyst trays.

The present invention is particularly suitable for the modification ofexisting reactors as can be seen from the following discussion. For anexisting reactor, a steady state flow simulation is conducted inaccordance with well known methods. In one embodiment, a cold flow isconducted using a two-dimensional axis-symmetric reactor model. Thegeometry is divided into about 14,000 hexahedral cells. Average valuesfor temperature, molecular weight and specific heats were utilized fromactual historical data at the affected plant. Table 1 below showstypical operating conditions.

TABLE 1 Flow, lb./h 500,000 Exit pressure, psia 8-14 Averagetemperature, ° F. >1000 Average molecular weight 25.9-26.2 Averageviscosity, cp 0.003 Average specific heat, J/Kg. ° C. 2400-2500

Catalyst bed porosity was also considered. This took into account theshape and particle size of the catalyst. Various catalysts wereconsidered, including smooth, shaped and ribbed versions. Catalystpellet diameter ranged from 3-3.5 millimeters (mm) with pellet lengthfrom 4-9 mm. The catalyst bed densities varied from 70 to 95 pounds percubic foot. The catalyst bed porosity changes during reactor operations,achieving its lowest value at end-of-run operation.

The velocity profile and pressure was calculated for each cell withinthe reactor geometry. Calculations for fluid flow simulations are wellknown in the art and the methods to achieve such are not subject of thisinvention.

Flow distribution for a reactor similar to that shown in FIG. 1, butwithout baffles 30, 31 and 32, showed that the top 20% and the bottom20% of the reactor bed operated at higher than normal LHSV. On the otherhand, the middle 60% operated at lower than normal LHSV. Thus, flow as afunction of distance decreased in the bottom half of the reactor andincreased in the top half of the reactor. The increase in the flow inthe top half is attributed to the higher pressure drop in the outerannulus as compared to that in the inner annulus. At the bottom half,the increased flow was attributed to excessive inner annulus pressuredrop as compared to the outer annulus.

It was determined that for a retrofitting of an existing reactor, thebest approach to correct the fluid flow was to place baffles ofspecified size at specific heights on the displacement cylinder toprovide as close to uniform flow through the reactor. Adding rings orbaffles in the top section of the inner annulus increased,the innerannulus pressure drop to match the pressure drop in the outer annulus.Catalyst bed fluidization was taken into account in the placement andnumber of baffles or rings. Some of the requirements of the presentinvention include the optimization of pressure drop in addition tominimization of pressure drop. In other words, the addition of bafflesincreases the pressure drop in the reactor. The addition of the bafflesshould be done with minimal increase in pressure drop and maximum effecton fluid flow normalization. Additionally, it is preferred that thebaffles extension into the reactor is limited to not exceed half thedistance from the outer wall on the displacement cylinder to the innerwall of the catalyst bed, i.e., half the width of the inner annulus.This combination of factors is included in the simulation solution forfluid flow normalization.

The addition of baffles or rings on the outer surface of thedisplacement cylinder (cylinder 12) allows the modification of thereactor fluid flow without having to take out the displacement cylinderor the catalyst trays or structure. The displacement cylinder is ofsufficient width that a man way is cut into the top and work isperformed from the inside of the displacement cylinder. The cylinderwall is cut at the required location and the baffles are added andwelded from the inside. This does not require any dismantling of thereactor and the changes can be made with minimum down time.

In accordance with one embodiment of the present invention, an existingethylbenzene dehydrogenation reactor was analyzed and modified. Thereactor space was 62 feet in height with a 60 inch diameter inlet and an88 inch diameter outlet. The reactor had an inside diameter of 13.5feet. The outside diameter of the displacement cylinder was 5.75 feet.The catalyst bed had an inner diameter of 7 feet and 5.25 inches and anouter diameter of 12.5 feet. The reactor had an outer annulus of 6inches depth. In operation, this reactor had a flow rate of over 500,000pounds per hour. As can be seen from the dimensions, the available spaceinside the reactor does not allow room for individuals to work in theavailable space. Any modification to the system would requiredismantling of the reactor. This has been the case until the presentinvention. To improve the fluid flow of this reactor according to itsoperating parameters, three baffles or rings were installed on the outersurface (reaction side) of the displacement reactor. The baffles areshown as items 30, 31 and 32 in FIG. 1. The top ring 30 was installed ata height of 49 feet and extended 7.5 inches into the reactor. The middlering or baffle 31 was installed at a height of 46.75 feet and extended6.25 inches into the reactor. The bottom ring 32 was installed at aheight of 40 feet and extended 5.0 inches into the reactor. Thesebaffles resulted in a more uniform flow of fluid through the reactor. Ascan be seen, the baffles were added to the top half of this reactor withincreasing extension to the reactor as height in the reactor isincreased. The improvement in the fluid flow uniformity resulted inincreased catalyst life and efficiency.

FIG. 2 shows the effect of adding the baffles in the above example. Thesolid line shows the fluid flow in the vertical direction through thereactor prior to the addition of the baffles. The graph shows the abovenormal fluid flow at the lower part of the reactor and the top part. Theaddition of the baffles resulted in normalizing the flow at the top(location of the baffles) and improving fluid flow through the middle.

Thus, the present invention, as disclosed in the aforementioned drawingsand descriptions corresponding thereto, provides means and apparatus forthe dehydrogenation of ethylbenzene to styrene, which process andapparatus enjoys the advantages of extended catalyst life and closercontrol of flow velocities at various points up-and-down the reactorcross-sectional configuration. Conventional reactors suffer from shortcatalyst life due to non-consistent flow velocities across varyingsections of the catalyst beds.

It was also discovered that flow velocities through the top of thecatalyst bed were in the range of one and one-half to two and one-halftimes higher than those across the middle of the bed. Thus, it wasrealized that utilization of the catalyst in the reactor was far fromuniform, which in turn contributed directly to much shorter thanexpected catalyst life.

As a result, the present invention discloses reactor configurations thatsignificantly reduce the vertical flow velocity variations. This isachieved by the use of baffles along the exterior wall of thedisplacement reactor. Flow simulations are utilized to determine thenumber, location and size of the baffles.

In typical operation, ethylbenzene feedstock is supplied to the reactorvessels via feed supply line 13 through inlet area 14. From there thefeed material flows into the reactor around the catalyst beds 18.Operating conditions in the reactor are preferably in the range of about900° to 1225° F. temperature, and about 8-22 PSIA pressure. Flowvelocities in the reactor range from about 100 to 400 fps, with apreferred overall flow velocity through the reactor of around 200 to 300fps.

Although certain preferred embodiments of the present invention havebeen herein described in order to provide an understanding of thegeneral principles of the invention, it will be appreciated that variouschanges and innovations can be effected in the described dehydrogenationreactor assembly without departing from these principles. For example,whereas the preferred embodiment is described as adding three baffles atthe top half of the reactor, the number and location of the baffles mayvary depending on the particular reactor. Also, it is apparent thatdifferent baffling shapes could be utilized to achieve flownormalization. Other changes would be apparent to one skilled in the artand therefore the invention is declared to cover all changes andmodifications of the specific examples of the invention, hereindisclosed for purposes of illustration, which do not constitutedepartures from the spirit and scope of the invention.

What is claimed is:
 1. A process for improving fluid flow uniformity ina gas phase reactor comprising an outer reaction vessel, and innerdisplacement cylinder having a top half and a bottom half and a reactionouter surface and an inert inner space, and an annular catalyst bed, theprocess comprising: conducting fluid flow simulations using actualreactor conditions; adding baffles on the outer reaction surface of thedisplacement cylinder to improve simulated fluid flow; and adding thebaffles to the displacement cylinder by entering the inner inert spaceof the cylinder and attaching the baffles to the reaction outer surfacefrom the inner inert space.
 2. The process of claim 1 wherein threebaffles are added to the top half of the displacement cylinder andwherein said baffles are added without disaddembly of the reactor orcatalyst bed.
 3. The process of claim 1 wherein the annular catalyst bedis at a certain distance from the displacement cylinder and wherein thebaffles extend into the reaction vessel by a distance not greater thanhalf the distance to the catalyst bed.